1. Field of the Invention
The present invention relates to an aspherical microstructure, such as an aspherical microlens array that is usable in fields of optoelectronics and the like, a mold or a master of a mold (in the specification the term xe2x80x9cmoldxe2x80x9d is chiefly used in a broad sense including both a mold and a master of a mold) for forming an aspherical microstructure, a method for fabricating the aspherical microstructure, and so forth. In the specification, the term xe2x80x9casphericalxe2x80x9d is used in a broad sense and includes xe2x80x9cacylindricalxe2x80x9d.
2. Description of the Related Background Art
A microlens array typically has a structure of arrayed minute lenses each having a diameter from about 2 to 3 microns to about 200 or 300 microns and an approximately-semispherical profile. The microlens array is usable in a variety of applications, such as liquid-crystal display devices, light receivers and inter-fiber connections in optical communication systems.
Meanwhile, earnest developments have been made with respect to surface emitting lasers and the like that can be readily arranged in an array form at narrow pitches between the devices. Accordingly, there exists a significant need for microlens arrays with narrow lens intervals and a large numerical aperture (NA).
Likewise, light receiving devices, such as a charge coupled device (CCD), have been reduced in size as semiconductor processing techniques develop and advance. Therefore, also in this field, the need for microlens arrays with narrow lens intervals and a large NA is increasing. In the field of such a microlens, a desirable structure is a microlens with a large light-condensing efficiency which can highly efficiently utilize light incident on its lens surface.
Further, similar desires exist in the prospective fields of optical information processing, such as optical parallel processing-operations and optical interconnections.
Furthermore, display devices of active or self-radiating types, such as electroluminescent (EL) panels, have been enthusiastically studied and developed. Highly-defined and highly-luminous displays have been proposed. In such a display, there is a heightened desire for microlens arrays with improved luminescence and visibility, a large area, a small lens size, and a large NA, and which can be produced at a relatively low cost.
In general, the spherical aberration of a lens is likely to increase as its NA is enlarged while its spherical profile is maintained. Therefore, the surface of the lens needs to be aspherical such that compensation for the spherical aberration can occur. Similarly, in the case of a microlens, the profile of the microlens should be aspherical to improve its light-condensing efficiency. The need to improve the light-condensing efficiency is caused by the downsizing of optical devices.
In such a situation, the need for methods for fabricating aspherical microlens arrays and aspherical microlens arrays produced by these methods is increasing.
A conventional method for fabricating aspherical microlens arrays has been proposed. In a mask layer formed on a flat surface, openings are formed at positions of individual lenses and their surrounding positions. Through those openings, parts of the flat surface are chemically etched to form a mother substrate. Then, a mold for forming an aspherical microlens array is fabricated by using the mother substrate (see Japanese Patent Application Laid-Open No. 5(1993)-150103). However, when an isotropic etching using a chemical reaction is employed, etching of the substrate into a desired profile cannot be achieved if the composition and the crystalline structure of the substrate vary even slightly. In addition, etching will continue unless the substrate is washed immediately after the desired shape is obtained. When an aspherical microlens array must be precisely formed, deviation from the desired shape caused by excessive or over etching should be considered.
Further, another conventional method for fabricating aspherical microlenses has been proposed. Lens mold elements are tentatively formed in a lattice array by a punch having the desired radius of curvature for the lens mold element. The desired radius of curvature is obtained beforehand. The surface height error of a lens mold element surrounded by other lens mold elements is measured radially from the lens mold element, and the error is substituted by a correction amount. The radius of curvature of the punch is corrected based on that correction amount by a numerical control (NC) abrasion machine. Then, a microlens mold with a desired aspherical surface is accurately formed using the corrected punch (see Japanese Patent Application Laid-Open No.6(1994)-154934).
In this method, however, much time is required to form a large number of concave molds over a large area. Further, it is difficult to fabricate concave molds with a uniform profile on the surface of the substrate when the punch""s durability is insufficient.
Furthermore, still another conventional method for fabricating aspherical microlens arrays has been proposed. First, a photosensitive resin is deposited on a substrate. Next, an array of cylindrical resin layers are formed, in accordance with the required lens number and the intervals of a lens array, by using photolithography. Then, the cylindrical resin layers are thermally processed to deform them into spherical resin layers. The substrate is dry-etched using the spherical resin layers as a mask, and a profile similar to the profile of the mask is transferred onto the substrate. Then, the thus-fabricated mother substrate is used as a mold (see Japanese Patent Application Laid-Open No.7(1995)-104106). In this method, an aspherical microlens array can be provided when the dry etching is performed such that the etching rate of the substrate is greater than the etching rate of the mask.
In this method, however, it is difficult to fabricate an aspherical microlens array with a large fill-factor since the dry etching is conducted using disk-shaped resin masks and a flat portion remains on the etched mother substrate.
It is an object of the present invention to provide a method of fabricating an aspherical microstructure, which can readily fabricate and control the aspherical profile of the microstructure, and increase the fill-factor and the size of the aspherical microstructure; a method of fabricating a mold used to form an aspherical microstructure, such as an aspherical microlens array, an aspherical fly-eye lens, and an aspherical lenticular lens; a method of fabricating an aspherical microstructure using the mold; and an aspherical microstructure. More particularly, it is an object to provide a mold for forming an aspherical microlens array. It is an additional object to provide a method of fabricating such a mold, and a method of fabricating the aspherical microlens array using the mold.
The present invention is generally directed to a method of fabricating an aspherical microstructure, which includes a step of forming a protruding microstructure on a substrate, a step of forming an aspherical-profile forming layer on the substrate and the microstructure, and a step of hardening the aspherical-profile forming layer. An aspherical profile is formed on the microstructure due to a surface tension of the aspherical-profile forming layer during the step of forming the aspherical-profile forming layer, and the aspherical profile is maintained in the step of hardening the aspherical-profile forming layer.
With this method, a curvature of the microstructure can be increased and the aspherical profile can be readily fabricated and regulated, since the protruding microstructure is formed on the substrate using electroplating, for example, and the aspherical profile is formed on the protruding microstructure utilizing the surface tension of the aspherical-profile forming layer. Further, the fill-factor can be increased by forming another layer on the aspherical-profile forming layer, and an array of aspherical structures with a large area can also be constructed.
More specifically, the following constructions can be preferably adopted based on the above fundamental construction.
The step of forming the protruding microstructure can include a step of preparing a substrate with a conductive portion, a step of forming an insulating mask layer on the conductive portion of the substrate, a step of forming at least one opening in the insulating mask layer to expose the conductive portion at the opening, and a step of forming an electroplated or electrodeposited layer in the opening and about the insulating mask layer around the opening using electroplating or electrodeposition. This method of forming the protruding microstructure can make it easier to obtain a microstructure with a large curvature and an array structure with a large area. With this method, the opening can have a circular shape or a slit shape, a plurality of openings with a common shape can be formed in the insulating mask layer, a plurality of regularly arranged openings can be formed in the insulating mask layer, and the electroplated or electrodeposited layer can have an approximately-semispherical shape or an approximately-semicylindrical shape. Those shapes are appropriately determined according to the application of the microstructure.
The aspherical-profile forming layer may be formed after the insulating mask layer is removed. When the insulating mask layer has no adverse influences, that layer need not be removed.
Here, electroplating and electrodeposition will be described with reference to FIG. 9. Where the electroplating is effected at a minute opening (which is formed in an insulating mask layer 51 formed on a conductive layer 50 on a substrate 49) in an electroplating solution 53 containing ions such as metal ions, metal ions in the electroplating solution 53 move toward the opening. Deposition of the electroplating proceeds with its growth direction being isotropic, as illustrated in FIG. 9. Thus, a spherical or cylindrical layer 52 can be formed. When the size (diameter or width) of the opening is sufficiently smaller than the size of an anode plate (not shown) and metal ions are uniformly dissolved in the electroplating solution 53, the growth direction of the plated layer 52 is isotropic. Each aspherical microlens fabricated can have any size when applying the above process.
The size of the opening should be smaller than the desired diameter of the desired aspherical microlens. In order to better achieve an isotropic growth of the plated layer 52, the size of the opening is less than the diameter of the spherical structure. The size of the opening should be determined while considering the size of the aspherical microlens to be fabricated.
In the case of electroplating, the spherical plated layer is formed by the deposition of metal ions in the electroplating bath caused by the electrochemical reaction. The amount of the plated layer deposited can be readily controlled by controlling the plating time, the temperature of the electroplating liquid, the amount of current, and so forth. The following materials can be used as electroplating metal, for example. As a single metal, Ni, Au, Pt, Cr, Cu, Ag, Zn and the like can be employed. As an alloy, Cuxe2x80x94Zn, Snxe2x80x94Co, Nixe2x80x94Fe, Znxe2x80x94Ni and the like can be used. Any material can be used so long as electroplating is possible. Ni, Cr, and Cu are especially preferable as electroplating materials for molds for aspherical microlens arrays since those materials make it possible to readily perform a gloss plating.
With respect to the size and so forth, electrodeposition is the same as above. As electrodeposition liquids, electrodepositable organic compounds (acryl-series resin and the like in the case of the anionic-type electrodeposition, and epoxy-series resin and the like in the case of the cationic-type electrodeposition) can be used.
Alternatively, the step of forming the protruding microstructure can include a step of forming a reflow layer in a desired pattern on the substrate, and a step of performing a reflow-processing of the reflow layer to form the protruding microstructure. The reflow layer subjected to the reflow-processing can have an approximately-semispherical shape or an approximately-semicylindrical shape.
The number of the protruding microstructures can be single or plural. The plurality of the protruding microstructures can have a common profile, and can be regularly arranged. The protruding microstructure can have an approximately-semispherical shape or an approximately-semicylindrical shape.
The aspherical-profile forming layer can be formed using a spin-coat method, a dip-coat method, or a chemical vapor deposition (CVD) method. The aspherical-profile forming layer can also be formed of a plastic material by using the CVD method, and deformed into an aspherical profile in a thermoplastic manner. The aspherical-profile forming layer can be formed of an organic material, an inorganic material, a metal, a metal compound, or the like. That material is appropriately determined according to the application of the microstructure. The aspherical-profile forming layer can be formed of such a material, so far as the material can have a desired surface tension, can be hardened, and, if necessary, has no adverse influences on the formation of a fly-eye forming layer.
Further, when a CVD method employing a plastic material is used, the aspherical-profile forming layer is processed such that a surface tension can act on its surface profile and the aspherical-profile forming layer formed by the CVD method can have a desired aspherical profile. In this case, the aspherical profile can be controlled by processing conditions, processing time, and so forth. The fabrication method of the aspherical-profile forming layer is not limited to the above method. For example, a spray-coat method can be used so far as a material with no adverse influences on the formation of the fly-eye forming layer can be deposited with a desired surface tension. In this case, the aspherical profile can be controlled by the spray amount, a viscosity coefficient of a spraying liquid, and so forth.
The thus-fabricated substrate with the aspherical profile can be directly used as a mold for an aspherical microlens array or the like. Alternatively, the thus-fabricated substrate can be used as such a mold after the surface of the substrate is coated with a material of Cr or the like having excellent chemical stability and mechanical strength.
The fabrication method of this invention can further include a step of forming a fly-eye forming layer on the aspherical-profile forming layer to form a fly-eye construction on a plurality of the protruding microstructures. In this case, the fly-eye forming layer can be formed until the fill-factor of the fly-eye construction reaches approximately 100%.
The fly-eye construction can be obtained by forming the fly-eye forming layer on the aspherical profile and joining adjacent sides of the protruding microstructures along the diagonal direction. Thus, even a mold for an aspherical microlens array with the fill-factor of about 100% can be fabricated.
The fly-eye forming layer can be formed of an organic material, an inorganic material, a metal, a metal compound, or the like, so far as a material of the layer can be used in an isotropic-growth method and can be employed as a mold. Also in this case, chemical stability and mechanical strength of the thus-fabricated mold can be improved by coating the surface of the mold with a material of Cr or the like having excellent corrosion and abrasion resistivities. The fly-eye forming layer can be formed using an electroplating method, an electroless plating method, a CVD method, or the like. The fabrication method of the fly-eye forming layer can be determined depending on a material of the aspherical microstructure.
For example, when the electroplating method is used, the aspherical-profile forming layer itself can be used for the electroplating if this layer is conductive. Where the aspherical-profile forming layer exhibits no conductivity, a conductive layer is formed on the layer using a sputtering method, a resistor heating method, an electron-beam evaporation method, a CVD, an electroless plating, or the like. Thus, electroplating can be performed. When the fly-eye forming layer is isotropically grown by the electroplating, the fly-eye construction can be formed with the profile of each protruding microstructure being maintained aspherical.
When the electroless plating method is used, the aspherical-profile forming layer itself can be used for the electroless plating if this layer is suitable for the electroless plating. When the aspherical-profile forming layer is unsuitable for the electroless plating, the substrate surface needs to be subjected to an activation process with an activating liquid, or a material suitable for electroless plating needs to be formed on the substrate using a sputtering method, a resistor heating method, an electron-beam evaporation method, a CVD, or the like. Thus, electroless plating can be performed. Also here, when the fly-eye forming layer is isotropically grown by the electroless plating, the fly-eye construction can be formed with the profile of each protruding microstructure being maintained aspherical.
When the CVD method is used, the aspherical-profile forming layer itself can be used for the CVD if this layer is resistant to CVD conditions. Also here, when the fly-eye forming layer is isotropically grown by the CVD method, the fly-eye construction can be formed with the profile of each protruding microstructure being maintained aspherical.
The substrate with the aspherical profile obtained by the above fabrication methods has the fly-eye construction, and this substrate can be used as a mold for an aspherical microlens array with the fill-factor of about 100% (i.e., without any optically unusable portions). An aspherical microlens array obtained by molding using this mold has the same profile as that of the mold.
The aspherical microstructure can be a mold for a microstructure array such as a mold for an aspherical microlens array. The substrate, the protruding microstructure, and the aspherical-profile forming layer can be formed of transparent material, respectively. In this case, the microstructure can be directly used as an aspherical microlens array.
In the present invention, a mold for an aspherical microlens array or the like can be fabricated by using the above aspherical microstructure as a master mold, and the profile of an aspherical microlens array or the like molded by using this mold is identical with the profile of the above aspherical microstructure.
The mold for an aspherical microlens array or the like can be fabricated directly on the above substrate using a method, such as a spin-coat method, a dip-coat method, a CVD method, an electroplating method, or an electroless plating method. Accordingly, even when its array increases, it can be readily fabricated with a large size. Further, the size and profile of the aspherical microstructure can be readily and precisely controlled at each fabrication step. Therefore, the lens shape and size of an aspherical microlens array fabricated by that mold can be readily and precisely controlled.
In those cases, the profile of the electroplated layer grown through the opening can be approximately-semispherical as illustrated in FIG. 9, so that the NA of the thus-obtained aspherical microlens can be enlarged.
The present invention is also directed to an aspherical microstructure which includes a substrate, a protruding microstructure formed on the substrate, and an aspherical-profile forming layer formed and hardened on the substrate and the microstructure. An aspherical profile is formed on the microstructure due to a surface tension of the aspherical-profile forming layer.
These advantages, as well as others will be more readily understood in connection with the following detailed description of the preferred embodiments of the invention in connection with the drawings.